Add effort to an extended test of skill which you passed.

You may use this ability for free once. Once you do, you cannot do so again until you rest.

Subtle:
Casting this spell while hidden also hides that you are casting it, and does not cause you to become unhidden.
When you cast this spell while you are not hidden, you can try to hide that you are casting it with a contest of Spellcraft or Roguery against the Seeking of creatures observing you. Only observers that win notice you are casting a spell. However, once the spell is cast, its effects may be noticed.